Bolted articulated piston

ABSTRACT

A piston ( 20 ) for an internal combustion engine includes an upper crown ( 22 ) and a lower crown ( 24 ) that are each fabricated using a same strong material, such as steel and fastened together using a plurality of fasteners ( 50 ). The piston ( 20 ) provides increased strength resulting in improved design flexibility and smaller sized engines that produce the same amount of power. Piston ring grooves ( 100, 102, 104 ) are also provided in either one or both of the upper and lower crowns ( 22, 24 ) which control piston combustion height. A deeper combustion bowl ( 72 ) is also available with the piston ( 20 ).

This application is a divisional of application Ser. No. 09/198,297,filed Nov. 23, 1998.

FIELD OF THE INVENTION

The present invention relates to a piston for large bore internalcombustion engines and in particular, to an articulated piston having acrown portion formed by connecting an upper crown to a lower crown usingfasteners.

BACKGROUND OF THE INVENTION

It is known to use pistons for large bore internal combustion engines.Typically, pistons have essentially two functional parts, a crown and askirt. There are two basic designs for large bore pistons, articulatingand non-articulating. Articulating pistons have separate skirts that canpivot about a wrist pin relative to the crown. In comparison,non-articulating pistons have skirts that are integral with the crown.It is also known to divide the crown into upper and lower portions thatare subsequently fastened together to form a three-piece piston. Thelower crown includes a pair of pin bosses which each have a pin bore toreceive the wrist pin.

Some non-articulating pistons are called composite pistons because theyhave a steel upper crown connected to an aluminum, or other light alloy,lower crown to reduce weight. Steel is employed in the upper crown tomaintain an adequate degree of strength in the head region, which isexposed to very high combustion temperatures and pressures. The steelupper crown is often connected to the aluminum lower crown usingprecision bolts.

The hybrid metal composition also sacrifices the overall strength of thepiston. During operation, pistons experience two separate loads. First,the crown experiences a vertical load from the combustion of the gasgathered in the combustion bowl at the top of the upper crown. It isimportant to select a material that is strong enough to withstand thecombustion load, for example, steel. Next, the skirt receives a sideload from the mechanical motion of the crankshaft. In non-articulatedpistons, the skirt is integral with the lower crown therefore, bothloads are imposed on the entire piston. As a result, the movement andperformance of the crown interferes with the movement of the skirt.

Also, hybrid steel/aluminum pistons typically have piston ring grooveslocated in the side walls of the upper crown because aluminum side wallsare not strong enough. If the ring grooves are located in the aluminumside walls, they must be reinforced with a stronger metal alloy toaccommodate the vertical load experienced during combustion.

The combined load on the crown and skirt also causes severe frettingwear in known bolted, non-articulated pistons, and reduces service life.Fretting wear occurs in generally stationary joints when very minoramounts of relative movement create microwelding between components,such as between an upper crown and a lower crown. Microwelds are formedand then broken in successive movements between the crown parts.Further, increased fretting wear occurs at the steel/aluminum interfaceincreasing the amount of contamination particles in precision enginesand reducing service life of the piston and engine.

The wrist pin is mounted in the pin bores of both pin bosses,constituting a bearing, and must be adequately supported against thepiston. Thus the pin boss is the pivot of the piston, the wrist pin andconnecting rod assembly. In highly loaded piston applications, aparticularly complicated design of the pin boss is needed when aluminumor other light metal alloys are used for the pin boss material.Incipient cracks can occur in the wrist pin if the maximum allowablespecific pressure is exceeded. To avoid such cracks in the wrist pin, itis known to profile the pin bore to relieve the stresses caused bydeformation of the wrist pin. In a non-articulated piston with analuminum lower crown, the thickness of the walls around the pin boresand precision profiling of the pin bore itself are necessary to overcomethe weakness of the aluminum material.

One alternative for attaching an upper crown to a lower crown isfriction welding. However, pistons of 250 mm diameters and more are veryheavy and are relatively low volume items. In addition, many frictionwelding machines do not have enough radial clearance to assemble largediameter pistons. Thus, it is not usually economically feasible toinvest in larger friction welding equipment to assemble low volume,large diameter pistons.

SUMMARY OF THE INVENTION

The present invention is directed to an articulated type of pistonhaving a crown portion formed by connecting an upper crown to a lowercrown using fasteners. By connecting the upper and lower crown withfasteners, the present invention has greater flexibility formanufacturing, piston design and locations of piston ring grooves.

In the present invention, the upper and lower crowns are made fromsimilar metals, preferably steel. Using steel for both portions of thecrown allows for increased flexibility in the location of the pistonring grooves. For example, the piston ring grooves can be located inboth the upper and lower crowns because steel does not requirereinforcement. By locating at least one compression ring groove in thelower crown, the compression height of the piston can be reduced. Thecompression height is defined by the distance from the top of the pistonto the center line of the pin bore. Reducing the piston compressionheight results in smaller engine designs that provide the same power ascorresponding larger engines. Smaller engines require smaller envelopesin vehicles and are lighter in weight.

It was generally accepted that similar materials have a tendency toattract and weld to each other on a microstructure level. In the presentinvention, however, fretting wear surprisingly decreased compared toknown hybrid non-articulating pistons.

In addition, the articulated piston configuration of the presentinvention allocates the vertical and side loads between the crown andthe skirt, individually. The present invention increases strength andloading providing ability to sustain higher peak combustion pressures.Generally, the weight of the present invention is comparable to thehybrid steel/aluminum type, but with potential to increase engineratings and lower overall engine weight by providing stronger pistons.

In the hybrid steel/aluminum configuration, the strength of the aluminumlower crown determined the depth of the combustion bowl. The steel upperand lower crown configuration of the present invention, however,increases the ability to form deeper combustion bowls.

In the present invention, using a steel lower crown reduces the need toprofile the pin bore because the steel material is strong enough toendure the stress caused by the piston pin.

The steel upper and lower crown configuration of the present inventionincreases the strength of the crown and the pin bores, the ease ofmanufacturing, and flexibility of the location of piston rings. Thus,the present invention provides a smaller overall engine with the samepower and less overall weight.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and inventive aspects of the present invention will becomemore apparent upon reading the following detailed description, claims,and drawings, of which the following is a brief description:

FIG. 1 is an exploded view of a piston assembly according to the presentinvention.

FIG. 2 is a sectioned perspective view of a piston assembly according tothe present invention.

FIG. 3 is a cross-sectional elevational view of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows an exploded view of a piston assembly 20 having an uppercrown or crown part 22 that is designed to be attached to a lower crownor crown part 24. A separate skirt 26 is also provided for articulatingconnection to lower crown 24 by a wrist pin (not shown). A pair ofbushings 28 are optionally installed into pin bores 30 in downwardlydepending pin bosses 31 to provide a bearing surface for engaging thewrist pin (not shown), which connects piston assembly 20 to a connectingrod (not shown). Upper crown 22 includes a plurality ofcircumferentially spaced apart bosses 32 having fastener bores 34 thatalign with corresponding bosses 36 in fastener bores 38 located on lowercrown 24. In addition, upper and lower crowns 22, 24 each have a pair oflocating bores 40 for corresponding alignment to receive locating pins42. Locating pins 42 aid during assembly to prevent rotation of uppercrown 22 relative to lower crown 24. In addition, locating pins 42 andlocating bores 40 serve as orientation indicators to ensure that if apiston assembly 20 is ever taken apart for service. Thus, upper crown 22will be oriented on lower crown 24 the same as before servicing.

A plurality of fasteners 50 are used to securely attach upper crown 22to lower crown 24. Fasteners 50 are shown in the form of studs 52,sleeves 54 and nuts 56. However, any suitable type of fastener can beused, including but not limited to, bolts, pins, studs, and screws.Studs 52 include upper and lower threaded ends 58, 60 with a necked downbody portion 62 in between. Sleeves 54 are designed to pass over lowerthreaded ends 60 and seat in lower crown bores 38. Nuts 56 are threadedonto lower threaded ends 60 and are tightened to engage and axiallypre-load sleeves 54.

Preferably, both upper and lower crowns 22, 24 are fabricated from thesame material to decrease the amount of fretting wear at the interfacejoint. Preferably, upper and lower crowns 22, 24 are both made fromsteel to provide increased strength and permit greater flexibility inpiston design.

FIGS. 2 and 3 show piston assembly 20 including upper crown 22 having anannular outer ring member 66 with a circumferentially extending outersidewall 68 and an uppermost surface 70. A combustion bowl 72 is locatedinterior of outer ring member 66 and forms a generally depressed recess74 having a very slightly convex top surface 76. Fabricating lower crown24 from steel instead of a weaker material, such as aluminum, increasesoverall piston strength and rigidity and permits combustion bowl 72 tohave a deeper design. If a lighter weight material is used for lowercrown 24, then the strength of the piston is reduced and would notpermit increasing the depth of combustion bowl 72 since thecross-sectional thicknesses would be reduced resulting in increasedflexing of pin bosses 31. Thus, combustion bowl top surface 72 can havea substantially increased depth D1 from uppermost surface 70. Forexample, depth D1 can be up to approximately 15% of the piston diameter,so a piston having an outer diameter of 250 mm can have combustion bowltop surface 76 located approximately 37 mm from uppermost surface 70.However, any suitable combustion bowl shape or configuration can be usedwith the present invention.

Upper crown 22 also includes an annular ridge or inner wall 80 thatdepends downwardly from a bottom surface 81 of combustion bowl 72 and iscircumferentially continuous. Bosses 32 are formed radially outwardlyand adjacent to ridge 80 to provide increased strength where fastenerbores 34 are located. In addition, an annular recess 82 is formedbetween outer sidewall 68 and combustion bowl 72 to define an upper partof a generally continuous cooling gallery 84.

Lower crown 24 has corresponding features that matingly engage withfeatures on upper crown 22. Specifically, lower crown 24 includes anannular recess 83, and an annular outer ring member 86 having an outersidewall 88. An uppermost surface 90 of ring member 86 is designed tomatingly engage ring member 66 of upper crown 22. Also, an annular ridgeor inner wall 92 projects upwardly for mating engagement with uppercrown annular ridge 80 and includes lower locating bores 40.Additionally, lower bosses 36 and lower fastener bores 38 are locatedradially outward of locating bores 40 and adjacent to annular ridge 92.A counter bore 94 is optionally provided in fastener bore 38 to receivesleeves 54. Preferably, counter bore 94 has a slightly larger diameterthan nut 56 to allow full compression loading of sleeve 54 to develop afully tensioned fastening system.

Using steel to fabricate both upper and lower crowns 22, 24 providesincreased flexibility in the location of piston ring grooves. Forexample, a first combustion ring groove 100 can be located on outersidewall 68 of upper crown 22. Second and third combustion ring grooves102, 104 can be located on sidewall 88 of lower crown 24 because thelower crown 24 is fabricated from high strength steel. An oilscraperring groove 106 is located below third combustion ring groove 104 andprevents oil from entering a combustion chamber (not shown). Althoughfour total ring grooves are shown, any suitable number of ring groovesare contemplated for the present invention. Moreover, the ring groovescan be located on either upper crown 22 or lower crown 24 or both. As aresult, compression height H can be reduced. The compression height isdefined by the distance from uppermost surface 70 to the center line ofpin bores 30. For example, a 250 mm diameter piston can be produced thathas a compression height of approximately 175 mm, or approximately 70%of the piston diameter. Reducing the piston compression height allowsfor smaller engine designs that provide the same power as correspondinglarger engines. Smaller engines also require smaller envelopes invehicles and are lighter in weight.

Skirt 26 surrounds pin bosses 31 and is preferably made of aluminum toreduce weight. However, any suitable material can be used.

Preferred embodiments of the present invention have been disclosed. Aperson of ordinary skill in the art would realize, however, that certainmodifications would come within the teachings of this invention.Therefore, the following claims should be studied to determine the truescope and content of the invention.

What is claimed is:
 1. An articulated piston for an internal combustionengine comprising: an uppermost crown part having an upper surface, alower surface, and an outer sidewall that defines an outer diameter; alower crown part having an upper surface, a lower surface and an outersidewall and at least one downwardly projecting pin boss for receiving awrist pin; said uppermost and lower crown parts being fabricated fromferrous-based metallic material and joined together by a plurality offasteners and including a separate skirt member fabricated ofaluminum-based material coupled in articulated manner to said lowercrown part.
 2. The piston of claim 1 wherein said uppermost, and lowercrown parts are both formed from steel.
 3. The piston of claim 1 whereinsaid at least one fastener comprises a plurality of threaded fastenersspaced circumferentially apart.
 4. The piston of claim 1 wherein saidlower crown part includes at least one combustion ring groove providedin one of said outer sidewalls.
 5. The piston of claim 1 wherein saidskirt member pivots relative to said lower crown part.
 6. The piston ofclaim 1 including an upper annular recess in said uppermost crown partlocated in opposing relation to a lower annular recess in said lowercrown part to form a generally continuous cooling chamber.
 7. The pistonof claim 1 further including a boss having a fastener bore for receivingsaid at least one fastener.
 8. The piston of claim 1 wherein saiduppermost crown part includes a lower surface including a downwardlyprojecting, generally annular ridge having at least one locatingfeature.
 9. The piston of claim 8 wherein said lower crown part includesan upper surface including an upwardly projecting, annular ridge formating engagement with said upper ridge of said uppermost crown part.10. The piston of claim 9 wherein at least one of said upper and lowerridges includes a boss located adjacent thereto for forming a fastenerbore.
 11. The piston of claim 10 wherein said boss is located radiallyoutwardly of said at least one of said upper and lower ridges.
 12. Apiston for an internal combustion engine comprising: an uppermost crownpart having an upper surface defining a combustion bowl and an outersidewall having a first combustion ring groove located therein; a lowercrown part having an annular ridge with an outer sidewall and at leastone downwardly projecting pin boss for receiving a wrist pin; a secondcombustion ring groove located on said outer sidewall of said lowercrown part; an oilscraper ring groove located on said outer sidewall ofsaid lower crown part; said uppermost and lower crown parts beingfabricated from ferrous-based metallic material and joined together by aplurality of fasteners and a separate skirt member fabricated ofaluminum-based material coupled in articulated manner to said lowercrown part.
 13. The piston of claim 12 wherein said uppermost and lowercrown parts are both formed from steel.
 14. The piston of claim 12wherein said at least one fastener comprises a plurality of threadedfasteners spaced circumferentially apart.
 15. The piston of claim 14including a plurality of bosses each having a fastener bore forreceiving said plurality of threaded fasteners.
 16. The piston of claim12 wherein said skirt member pivots relative to said lower crown part.17. The piston of claim 16 including an upper annular recess in saiduppermost crown part located in opposing relation to a lower annularrecess provided in said lower crown part to form a generally continuouscooling chamber.
 18. The piston of claim 12 wherein said uppermost crownpart has a lower surface including a downwardly projecting, generallyannular ridge having at least one locating feature.
 19. The piston ofclaim 18 wherein said lower crown part has an upper surface including anupwardly projecting, annular ridge for mating engagement with said upperridge of said uppermost crown part, at least one of said upper and lowerridges having a boss located radially outwardly and adjacent thereto forforming a fastener bore.
 20. An articulated piston constructioncomprising: an uppermost crown part fabricated of a ferrous-basedmaterial having circumferentially extending inner and outer walls spacedradially from one another; a lower crown part fabricated separately fromsaid uppermost crown part of a ferrous-based material, said lower crownpart having a pair of pin bosses with an aligned pin bore andcircumferentially extending inner and outer walls spaced radially fromone another; a separate skirt joined in articulated manner to said pinbosses of said lower crown part; and a plurality of circumferentiallyspaced fasteners joining said inner wall of said lower crown part tosaid inner wall of said uppermost crown part to unite said uppermost andlower crown parts and to provide a circumferentially extending coolinggallery between said inner and outer walls of said uppermost and lowercrown parts.
 21. The piston of claim 20 wherein said skirt is fabricatedof aluminum.